Electric discharge lamp



Dec. 27, 1938. I (:Q)( 2,142,047

ELECTRIC DISCHARGE LAMP Filfid Oct. 29,- 1935 ATTORNEY Patented Dec.- 21, 1938 UNITED STATES PATENT OFFICE ELECTRIC DISCHARGE LAMP Application October 29, 1935, Serial No. 47,258

2 Claims.

This invention relates to electriclamps and more particularly to lamps wherein the light is .produced mainly by an electric discharge having the characteristics of an are through an ionizable 5 medium.

The invention is in the nature of an improvement in the type of lamp disclosed in application Serial N 0. 758,405, filed December 20, 1934. There is disclosed in said application a lamp consisting in general of an enclosing envelope containing a filling of an ionizable medium which at the normal operating temperature of the lamp is at a relatively high pressure. Mounted within the envelope are two spaced electrodes of refractory l metal between which an arc is maintained when the lamp is operating. In order to facilitate starting of the lamp there is provided an auxiliary electrode in the form of a wire or mesh'which is mounted on the outer wall of the envelope. This 50 auxiliary electrode as well as other metal members may be in contact with the external wall of the lamp envelope, and I have found that the regions of the envelope adjacent these metal members become blackened or otherwise light 55 obstructive when the lamp is in use. Accordingly, it is one of the principal objects of this invention to provide means to prevent this blackening of the lamp envelope. 7

v I have further found that in an arc lamp of 0 the gaseous or vapor dischargetype particularly one of the metallic vapor type, there is a weakening of the lamp envelope in the regions where the auxiliary starting electrode or other metal members contact with the envelope, and in some 5 cases this action is suflicient to render the envelope either porous or brittle resulting in failure of the lamp. This is particularly true where the composition of the glass of the envelope contains one or more metals which are electrolytically ac- 0 tive with respect to the metallic vapor filling such for example as mercury vapor, sodium vapor, cadmium vapor and the like. Glass like any electrolyte will passcurrent if voltage is applied to it. The magnitude of this current varies with the 5 potential applied, thickness of the glass and also the temperature. The last of these variables is important in the type of lamp disclosed herein,

because the conductivity roughly doubles for each increase of 10 C. in temperature and the 9 glass in the mercury lamp operates at the unusually high temperature of 450 C. Another important factor in this electrolysis is the conditions within the envelope. Normally whenelectrolysis takes place there is an accumulation of decomposition products at the electrode-electrolyte surface. This is commonly called polarization and tends to stop further electrolysis. In the type of lamp with which this invention is concerned, on the inner surface of the bulb, there is the intense ionization of the discharge which not 5 only gives a low resistance path electrically but also is very hot and chemically active gas. These factors tend to pass the products of electrolysis into the gaseous atmosphere and to deposit them throughout the inner surface of the bulb. Acl0 cordingly, it is another object of the invention to provide an arc discharge lamp of the metal vapor "type with an external starting electrode together with means for preventing electrolytic or similar electrochemical action between the starting elec: 16 trode or its supports, and the vapor ,fllling.

A feature of the invention relates to an are discharge lamp of the metal vapor type having an external auxiliary starting electrode which is electrolytically insulated from the lamp envelope.

Another feature relates to the novel organization, arrangement and relative location of parts which go to make up an improved and eilicient lamp of the arc discharge type.

Other features and advantages not specifically 2 enumerated will be apparent after aconsideration of the following'detailed descriptions and the appended claims.

While the invention will be disclosed herein as embodied in one particular structure of arc discharge lamp it will-be understood that this is done merely for explanatory purposes and not by way of limitation. Accordingly in the draw- F ig. l is a view partly in section, of a lamp em- I bodying features of the invention.

Fig. 2 isa cross-sectional view of the lamp of Fig. 1 taken along the line 22 thereof.

Fig. 3 is a view, of part of a lamp embodying a modification of the invention.

Fig. 4 is a plan view, partly sectional, of one of the lamp supporting rings of Fig. 3.

Fig.5 illustrates another modification of the invention.

Referring to the drawing, the numeral I represents a. main enclosing envelope of transparent material such for example, although not necessarily, of combustion glass tubing. Sealed through appropriate seals into opposite ends of the envelope I, are the electrodes 2, 3 preferably although not necessarily of tungsten or similar refractory metal having a portion or all of their surfaces provided with an electron emissive coating such for example as an alkaline earth oxide or mixture of such oxides. The envelope I is first evacuated by well-known lamp exhaustion procedure and is then provided with a filling of an ionizable medium such for example as mercury, sodium, cadmium and the like. Preferably suilicient metal is placed within the envelope so that when the lamp is at its normal operating temperature, the pressure of 'the metal vapor is of the order of from 0.5 to 1.5 atmospheres and the'applied voltage is such as to maintain an arelike discharge between the electrodes. If desired, in order to facilitate starting, a small percent of a conductive inert gas such as argon, helium or the like may also be added to the filling.

The main envelope l is supported within an outer enclosing envelope 4 of any suitable trans- .parent glass having at one end any well-known form of contact terminal; While the drawing shows a terminal of the well-known screwthreaded type it will be understood that other forms of terminals may be employed. As shown in the drawing the envelope 4 is provided with the usual reeritrant stem 5 terminating in the press 6 into which are sealed the various current lead-rin wires and the wires for supporting the main envelope I. Thus there is shown sealed into the press 6 a wire I to which is fastened as for example by welding, a wire frame consisting of the uprights 8, 9 which frame is electrically connected as by the jumper wire it! to the upper electrode 3. Another wire (not shown) is sealed into the press 6 and is electrically connected by jumper wire II to the electrode 2. Wound or wrapped around the envelope i and welded or otherwise connected to the frame members 8, 9 is a wire mesh l2 which serves as an auxiliary starting electrode for the lamp. It will be understood.

that while the drawing shows the electrode i2 in the form of a wire mesh, any other well-known form of starting electrode may be employed. Thus this electrode may take the form of a single fine wire wrapped helically around the envelope l and in electrical contact with the frame members 8, 9. In any event, the auxiliary electrode is sumciently foraminous as not to interfere ma- {.erially with the light that is produced by the amp.

Preferably the envelope l is resiliently mounted in spaced relation to the wall of envelope 4 by any suitable form of resilient support. Thus as shown in Fig. 1, the frame members 8, 9 are welded or otherwise fastened to circumferentially flexible metal supports each support consisting for example of an inner ring l3 and an outer ring II, the inner rings contacting with the en velope l as shown. For a more detailed description of these resilient supports reference may be had to application Serial No. 758,405.

when a lamp of the above-described type is in operation there is produced between the electrodes 2, 3 a high temperature are and the heat of the arc correspondingly raises the temperature of the envelope I. I have found that with this type of lamp when the frame members 8, 9 are in direct contact with the envelope I, there is produced a blackening of the said envelope in the regions of contact. I have also found that this blackening may be prevented or at least reduced to a substantially imperceptible amount by interposing an appropriate insulator between the said frame members and the envelope. Pref-- erably this insulator should be of such a material that is not electrochemically active with respect either to the metal vapor filling or to the metal of the frame members 8, 9. I have found that mica and certain insulators of the ceramic type are suitable for this purpose. Thus as illustrated in the drawing there is interposed between the frame members 8, 9 and the envelope i narrow strips l5, iii of mica which is preferably transparent. Instead of providing mica strips for this purpose, the frame members i, 9 may be provided with a. coating throughout their lengths of a ceramic insulator such for example as aluminum oxide or the like. I have found that this arrangement prevents the undesirable blackening of the envelope I, and while I do not wish to be restricted to any particular theory in explanation of this result, it is probably due to the fact that the mica or ceramic insulator eliminates the electrolytic passage of mercury, or other metal of the vapor filling, through the glass of the envelope i. Furthermore, when this envelope is made of special glass containing for' example, sodium, barium, potassium, etc., the mica or ceramic shield, prevents electrolytic or similar electrochemical action between the glass and the frame members. It will be understood that the invention is not limited to the application of the mica or ceramic shield to the frame members 8, 9 alone but that it may be applied to any other metal member which contacts with the envelope i or which tends to produce an electrolytic blackening of this envelope. Thus as illustrated in Figs. 3 and 4 the auxiliary starting electrode is in the form of relatively fine wire I'I carrying a coating of insulation l8 preferably, of a ceramic nature. As shown, the wire is wrapped helically around the envelope l andpreferably in contact therewith. The envelope I may be supported by a frame consisting of the side-rods i9, 20 similar to the corresponding side-rods I, 9 (Fig. 1), however in the embodiment of Fig. 3 the side-rods I9, 20 are preferably spaced from the envelope and are electrically connected to wire i'l. Welded or otherwise fastened to the side-rods I9, 20 are resilient ring supports similar to those of Fig. 1, one of these supports is shown in detail in Fig. 4 and includes an inner ring 2| of spring metal provided with an insulating covering 22 preferably of ceramic material. Suitably fastened to the ring 2| is an outer ring 23 of spring metal having a series of circumferential enlargements to engage the inner wall of the envelope I. With this arrangement there is no electrolytic action between any of the metal parts and the envelope because the side-rods are spaced from the envelope and the starting electrode as well as the end supports are electrolytically insulated from the envelope. It will be understood of course that the end support rings instead of being provided with a coating of insulation may be separated from the envelope by a layer of mica or other similar insulation.

Instead of employing an auxiliary starting electrode in the form of a wire mesh or wrapped wire there may be sealed into the envelope i one or more wires or probes 25 which may be electrically connected to the supportingframe siderods l9, 2| as illustrated in Fig. 5. Here again preferably the end support rings are provided with insulating layers or coatings as described in connection with Fig. 3.

Various other changes and modifications may be made without departing from the spirit and 'scope of the invention. a

a-metallic supporting frame for said envelope. and connected toone of said electrodes, said frame comprising a pair of side-rods spaced from said envelope and a pair of end support rings for said envelope, and an external starting electrode of foraminous material surrounding part of said envelope but spaced from said envelope and connected to the supporting frame.

2. In an electric lamp of the character del0 scribed, the combination of an enclosing envelope,

a pair of electrodes within said envelope. a metailic frame member for supporting said envelope,

said frame member being electrically connected to one of said electrodes, and means to electrolytically insulate said frame member from said envelope throughout its length, said insulating means being in the form of a layer of aluminum oxide between the frame member and the wall of the envelope.

JAMES L. COX. 

